What purpose is served by the waters above the firmament? Gervase of Tilbury declared that they supply the earth with dew.[828] Abelard said that there were two opinions on this subject.[829] The first was that the waters were originally placed in the heavens in order to be used in the Deluge. To this he was opposed, because the Psalms show that the waters were still in existence in David’s time, long after the Flood. If there had not been waters above the firmament in David’s time, how could the latter have sung: “Praise ye the Lord ... ye heavens of heavens and let all the waters that are above the heavens praise the name of the Lord”?[830] Abelard was more inclined to favor the second theory, that the waters were intended to temper the heat of the upper celestial fires. He felt, however, with more or less reason, that this entire problem presents great, if not insoluble, difficulties. _THE CONGREGATION OF WATERS_ There is an abundance of evidence that the authority of the Bible was invoked to support a theory that the waters beneath the firmament must constitute one unit, or “congregation of waters.” This view, as we saw in Chapter II, was based on the assertion in Genesis that God “gathered together the waters in one place.” Peter Abelard,[831] Peter Comestor,[832] and Hugh of St. Victor[833] all maintained that there are great subterranean reservoirs connected with the seas and rivers of the surface in such a way that the whole hydrographic system of the earth forms a single unit. Prior to the action of God in gathering them together these waters in the primordial chaos were supposed to have been disseminated in the form of vapor, which took up vastly more room than the liquid into which God’s power later concentrated them. CONNECTION BETWEEN SEAS AND RIVERS That many writers believed in the connection between the seas and the rivers and in the consequent unity of the waters is shown by numerous passages. Medieval thinkers, as we have seen, were constantly preoccupied by the doctrine of the microcosm, the theory that the human body includes all the elements which constitute the universe and is indeed in itself a miniature replica of the universe. This appears in a statement in the _De imagine mundi_ that the whole interior of the earth is filled with channels like the blood vessels that permeate the body.[834] Whenever and wherever a man digs into the ground he is sure to find water. A constant circulation is maintained between the ocean and the waters of the surface of the land through these passages and through the air.[835]